In recent years, gallium nitride (GaN) compound semiconductor light-emitting devices have drawn attention as short wavelength light-emitting devices. The gallium nitride based compound semiconductor light-emitting device is formed on various kinds of substrates, such as a sapphire single crystal substrate, an oxide substrate, and a III-V group compound substrate, by a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
The gallium nitride based compound semiconductor light-emitting device is characterized in that a small amount of current is diffused in the horizontal direction. Therefore, a current is applied to only a semiconductor immediately below an electrode, and light emitted from a light-emitting layer immediately below the electrode is shielded by the electrode, which makes it difficult to emit light from the light-emitting device to the outside. In addition, in the light-emitting device, a transparent electrode is generally used as a positive electrode, and light is emitted to the outside through the positive electrode.
In the related art, a transparent positive electrode composed of the transparent electrode has a laminated structure of a Ni or Co oxide layer and an Au contact metal layer. In addition, in recent years, a technique has been proposed which uses as a positive electrode having a laminated structure of a layer formed of a conductive oxide, such as ITO (In2O3—SnO2), and a contact metal layer having a very small thickness, thereby improving transparency and the emission efficiency of light from a light-emitting layer to the outside.
However, the external quantum efficiency of the light-emitting device is represented by the product of light emission efficiency and internal quantum efficiency. The internal quantum efficiency means the ratio at which the energy of a current applied to the light-emitting device is converted into light. The light emission efficiency means the ratio of light emitted to the outside to light generated from the inside of a semiconductor crystal.
In recent years, the internal quantum efficiency of the light-emitting device has increased up to about 70 to 80% by improving the state or structure of crystal, and high internal quantum efficiency with respect to the amount of current applied has been obtained.
However, in light-emitting diodes (LEDs) as well as the gallium nitride based compound semiconductor light-emitting devices, generally, the light emission efficiency with respect to the amount of current applied is very low. Therefore, it is difficult to emit a sufficient amount of light to the outside with respect to the current applied.
The reason why the light emission efficiency is low in the gallium nitride based compound semiconductor light-emitting device is that, since the light-emitting layer of the gallium nitride based compound semiconductor has a refractive index of about 2.5 that is considerably higher than that of air, which is 1, and a small threshold angle of about 25°, no light is emitted to the outside due to repeated reflection and absorption of light in the crystal.
In order to improve the light emission efficiency, a gallium nitride based compound semiconductor light-emitting device has been proposed in which a rough emission surface is provided to emit light at various angles, thereby improving the light emission efficiency (for example, Patent Document 1, i.e. JP-A-6-291368).
However, in the light-emitting device disclosed in Patent Document 1, since the rough emission surface is formed on a semiconductor material, a large load is applied to the semiconductor layer, and the semiconductor layer is greatly damaged. Therefore, even though the external quantum efficiency (light emission efficiency) is improved, the internal quantum efficiency is lowered. As a result, it is difficult to improve emission intensity.
Further, a method of roughening the semiconductor layer requires minute mask patterning. Therefore, the manufacturing process becomes complicated, which results in low manufacturing efficiency.
Furthermore, since an uneven pattern formed by the mask patterning method causes interference, constructive interference occurs at a specific wavelength.
The invention has been made in order to solve the above problems, and an object of the invention is to provide a gallium nitride based compound semiconductor light-emitting device having high light emission efficiency and a small variation in wavelength and a method of manufacturing the same.